There is considerable interest today in waterless lithography and both positive and negative printing plates are available from various sources such as Toray Industries of Japan. The image area of a waterless plate is a photopolymer similar to that used on conventional lithographic plates, but the non-image area is made from a siliconized rubber which is ink repellant.
Waterless lithography has a number of advantages and disadvantages. The principal advantage is the elimination of water, dampening fluids and fountain solutions from lithographic printing. Press start-up is achieved faster without water and dramatic savings of paper through reduced wastage offer considerable cost benefits. The waterless plate prints a slightly heavier film of ink than a conventional plate and this, coupled with the fact that the ink has not been diluted with fountain solutions leads to an increase in the density of color on the printed substrate. Moreover, the elimination of the fountain solution lowers the dot-gain of inks to the extent of 10-40%.
In addition, waterless lithographic printing permits reduced drying time and energy costs and may be carried out using relatively unskilled personnel whereas the difficulty in controlling the ink-water balance in conventional lithography necessitates the use of highly skilled pressmen who have the requisite expertise to properly match the printing ink and the fountain solution.
There are several disadvantages associated with waterless lithography such as susceptibility of the coating on the plate to scratching during handling. Hard surfaced substrates or those with high levels of lint and debris tend to damage the surface of the plate and therefore limit its life.
The most serious drawback of waterless lithography is that the absence of water during printing leads to the press running hotter. This increase in temperature has an adverse effect on the rheology of the ink which must be kept as high as possible to avoid problems of ink being picked up in the non-image areas--a problem known as "toning". The temperature of the plate cylinder at which background toning starts to appear is termed the Critical Toning Temperature (C.T.T.). The C.T.T. measurement is a good measurement of toning resistance, but is dependent on a number of factors such as press type, ink roller assembly, ink roller setting, printing speed, ink film thickness on the rollers, etc. In order to standardize the C.T.T. values, Toray Industries of Japan developed a testing device and procedure based on a "Davidson 700" press. The C.T.T. value measured by such device is termed the Critical Toning Index (C.T.I.) as a standard measure of toning resistance.
Waterless lithographic printing necessitates the use of inks with a high viscosity but with low tack. Typically an ink for use in waterless lithography will have a viscosity of 2 to 3 times that of a conventional ink in order to reduce the possibility of background toning. Ordinarily, high viscosity inks will have high tack; high tack will cause problems known as picking and piling. Therefore the waterless ink must be carefully formulated with appropriate resins so as to insure that the formulation will have a high viscosity and a low tack.
Since higher press operating temperatures result from the elimination of the fountain solution, multiple ink formulations or cooling of the vibrational rollers (and preferably also the plate) must be employed to avoid the background toning problem. If the presses are not chilled, three types of ink are required for use in waterless lithography: one for very cold temperatures in winter, one for average temperatures in the spring and fall and one for very warm temperatures in the summer. As would be expected, printers are reluctant to stock different inks during the year and must therefore retrofit their existing presses with thermostatic controls and large chillers, or purchase new presses having such controls and chillers or rely on conventional lithography.
Unless chilling of the rollers and plates is employed, the average temperature of the press during a medium-length press run will rise by 10.degree. to 15.degree. C. or more. Excessive press operating temperatures will result in background toning, increased dot gain and ink roller instability on light forms. On the other hand if, as a result of chilling, the press operates at too low a temperature, poor ink transfer to the substrate, mottling of solids and picking of the surface of the paper will result. Thus press operating temperatures must be carefully maintained in a narrow range between about 26.degree. C. and 31.degree. C.
The present invention allows printers to achieve the advantages of waterless lithography without the need for expensive equipment (and energy) to chill the rollers and plates. The fact that an ink in the form of a water-in-oil microemulsion utilized in conjunction with a waterless lithographic ink permits the press to run without chilling and without resulting in any significant background toning was totally unexpected, since it was always believed that the presence of any water would be detrimental to "waterless" lithography. Moreover, print quality comparable to that obtained by chilled, waterless lithographic printing processes is obtained with the ink of the present invention, notwithstanding that it was always thought that an ink containing emulsified fountain solution produces poorer print quality than the typical high viscosity waterless ink.
It should be noted that since the ink of the invention has a lower tack than the typical waterless ink, its usage results in reduced linting, picking and piling, thereby affording cleaner prints and increased plate life. Moreover, the C.T.I. of the present ink is not reduced even though the tack has been reduced well below the level associated with typical waterless inks.